Abstract：The Hetao plain of Inner Mongolia is one of the serious endemic arsenic poisoning areas in the world. The authors took Hanggin Rear Banner in western Hetao plain as a typical study area, where endemic arsenic poisoning constitutes a very severe problem. Studies show that As concentration is 7.7-34.6mg/kg in sediments of the high As groundwater area and As concentrations are relatively high in clay and mild clay. According to the results of the authors’ investigation, As is incorporated in amorphous iron oxides and is associated with sulfides and targeting acid volatile sulfurs, carbonates constitute the main association of sedimentary As, accounting for over 50% of total As. pH values of high As groundwater are 7.0-8.3, average Eh value is -155.1mV, average TDS value is 1.58 g/L, main hydrochemical types are Cl-HCO3-Na, Cl-Na and HCO3-Cl-Na, and As concentration ranges from 15.5 to 1093μg/L and is mainly represented by As(Ⅲ), high level DOC (0.73-35.76mg/L), HCO-3(283.75-1290.48mg/L), NH+4(0.27-10.48 mg/L) and minor nitrates and sulfate. Hanggin Rear Banner is also one of the serious endemic fluorosis areas, with fluoride content being 1.11-6.01mg/L in high F groundwater. Due to the presence of excessive As in most high F groundwater areas, there exists a coexistence phenomenon of high F groundwater and high As groundwater. A comprehensive analysis shows that high As groundwater was formed in a reductive environment in Hetao plain. Under reducing conditions, the reductive dissolution of Fe/Mn oxides (or hydroxide) from sediments in high As areas can release arsenic into groundwater. This is the main factor responsible for the formation of high As groundwater in the study area.